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This invention relates to receiving and focusing of a radiating wave field that propagates in a medium, specifically where that medium causes wave field distortions that degrade quality of focus. Example applications in the field of the invention include ultrasonic medical imaging, seismic prospecting, ultrasonic industrial inspection, radar, sonar, and optics.
An ideal medium for propagation of radiating waves is free space where electromagnetic waves propagate at precisely known speed without distortion. Devices that utilize wave propagation in a medium that is less perfect than this ideal, are subject to a variety of limitations. There are many ways that a medium can be imperfect. It can attenuate signals, it can have propagation speeds that are inaccurately known, and it can be inhomogeneous. Wave fields are focused using an aperture where the quality of focus depends on characteristics of the medium and characteristics of equipment that implements the aperture. A variety of measures are needed to compensate for medium effects to improve quality of focus, but an especially difficult problem is presented when the medium is inhomogeneous.
In discussing wave fields, it is common to speak of wavefronts to help in intuitive understanding of a very complicated physical process. A wavefront is defined to be a surface that contains points of equal phase. A simple wave field has many possible wavefronts, but only one is pictured. However, when this discussion refers to wavefront shape or to modifying wavefront shape, all possible wavefronts are assumed to be so shaped or so modified. References made here to a wavefront are intended to be references to a wave or a wave field by implication. Changing amplitude of a wavefront means that the wave system represented by that wavefront is proportionally changed in amplitude. The term phase is also somewhat inadequate where a single frequency is not utilized. Here a wavefront refers to points at a similar position in a function that causes a wave, such as a leading edge of a pulse or a first peak. For purposes of the present specification, wave speed is the speed of a point on the wavefront, under either of these definitions of wavefront. Wavefronts are frequently described here as spherical surfaces which means that they are sections of a surface of a sphere. When shown as a three dimensional drawing these wavefronts are indicated with a wire mesh that describes the surface. In two dimensional drawings they are indicated by a curved line which is intended to mean a spherical surface, unless otherwise stated.
Waves and wavefronts are a form of signals as are electrical voltage variations in wires and samples of voltages that are in computers. All these forms represent information.
Inhomogeneous problems are significant in ultrasonic imaging in a three dimensional volume such as the human body. It is known that sound speed variations in human tissue can cause significant wavefront distortion over an aperture extent. Such spatial variations tend to defeat the basic focusing function of the aperture because this focusing function depends on a predicted wavefront shape for points in the focus zone and well formed wavefront shapes for wavefronts that come from any point outside the focus zone. Wavefront distortion causes amplitude reduction of a focused signal to be reduced in amplitude, widening of a focal zone (a beam), and increased response for points outside the focal zone.
High resolution is critical to viewing disease processes, but it is widely believed in the field of ultrasonic imaging that aberration effects of inhomogeneous media would limit usefulness of high resolution devices. In ideal media, resolution improves with the use of shorter wavelengths or larger aperture transducers, but in human tissue, which is inhomogeneous, the use of such measures is expected to lead to aberrations that would prevent full benefits that might otherwise be realized (M. O""Donnell and P. Li, xe2x80x9cAberration correction on a two-dimensional anisotropic phased array,xe2x80x9d 1991 Ultrasonics Symposiun, p1190, IEEE). Although detailed study of past experimental work reveals that spatially uniform attenuation that strongly varies as a function of frequency is also a significant limitation of large aperture devices, the aberration problem remains as a significant barrier to development of high resolution ultrasonic systems.
Inhomogeneous conditions cause degradation of a response function, where that response function describes performance of a transducer system. There can be a transmitting response, which is a measure power as intensity as a function of a spatial dimension or there can be a receiving response, which is a measure of sensitivity as a similar function. Reciprocity usually applies so a transducer response is the same for either direction. Where there is a strong peak in the response function, a main beam is established. In receiver terms, the key degradation issue is the relative strength of a signal that comes from points within a focal zone as compared with the strength of an off-beam signal that comes from points outside the focal zone. Off-beam response is often called sidelobe response, though there can also be a grating lobe response. Terminology tends to be inadequate in practice. It becomes particularly problematic when it leads to use of analytical methods that were developed for ideal media.
Understanding of propagation in a medium that is inhomogeneous requires a meaningful description in terms that can be mathematically analyzed. A common model in the field of medical ultrasonic imaging addresses irregular sound speed variations, where such variations cause wave arrival times at a receiving aperture to deviate from the ideal shape. Arrival time is represented by wavefront shape as it is immediately approaching a receiving aperture. Sound speed variations over the propagation path cause wavefronts to distort, but if accurate time corrections could be ascertained to compensate for the variations of sound speed, the main beam response would be restored. FIG. 1(a) shows as reference an ideal wave 2 propagating from a source that is approximated as a point 11 to a receiver 3 in a homogeneous medium 1 without distortion to cause received signals 4. FIG. 1(b) is comparative illustration for imperfect media 8 that shows a wave perturbing effect 7 of a localized material 5 that varies wave speed, where compensation material 6 is inserted as a lump that would reverse the initial variation of wave speed. In ultrasonic systems, the actual compensating process would be handled as an electronic process after reception of received signals 4. In optical systems, the use of corrective material is a common way to correct for lens errors, which have much the same effect.
This comparative illustration of FIG. 1(b) also shows a blockage 9 that distorts the actual wave by leaving a gap 10 where wave amplitude is zero. For relatively large blockage shown the gap 10 projects geometrically such that the propagated wave 70 proportionately contains a similar gap 71. Artificially filling in the gap is not inconceivable in simple conditions where signals are simply provided to establish uniformity, but in general it is difficult to know what signal is needed. Repairing this gap by time corrections is not conceivable since there is no wave energy to work with. Though it might seem innocuous, the gap is a cause of distortion of significance that is comparative to an uncorrected speed distortion effect. Coherent relationships of multiple gaps make them far more significant than relatively random speed distortion effects.
While this depiction of the problem of FIG. 1 shows relatively large irregularities, there is concern for many small irregularities often exist in an actual imaging situation. Simple projection to portray propagation effects becomes less reasonable for small gaps.
The general approach of applying time adjustments to correct for sound speed variations is valid for sound speed variations that are sufficiently large and slowly varying, and sufficiently close to the receiving aperture. Small to medium localized speed variations that occur in large numbers tend to cause problems with this approach. Attempts have been made to accommodate irregularities that are significantly separated from the receiving aperture using a method called back-projection (Liu and Waag, xe2x80x9cCorrection of ultrasonic wavefront distortion using backprojection and a reference waveform method for time shift compensation,xe2x80x9d (Journal of the Acoustic Society of America, 96 (2) August 1994) to establish a surface where time adjustments would be valid, but even if proper time corrections were found which effectively enhance main beam response, the same corrections would only be approximately useful to control off-beam response. Even with these limitations, the time correction approach has been the primary approach, and the basis of many inventions, of the ultrasonic community for many years. A history of development work by the ultrasonic community is discussed in detail in the background given by Langdon et al. U.S. Pat No. 6,223,599. In spite of these widespread efforts, nothing has been sufficiently effective or efficient that it has been made available in commercial medical equipment.
Although the mainstream view is that time correction is required to compensate for aberrations, other approaches have been considered. Zhu and Steinberg suggested large 2D apertures for suppressing random effects (Zhu and Steinberg, xe2x80x9cWavefront amplitude distortion,xe2x80x9d Journal of the Acoustical Society of America 96 (1) July 1994, pp1-9). Ultimately, Zhu et al. U.S. Pat. No. 5,935,068 disclosed a compression algorithm for suppression of aberration effects, though this would primarily act on a different form of distortion that is variation of amplitude rather than simple time of arrival variations. Zhu et al. attribute variation of amplitude primarily to refraction effects. Langdon et al. U.S. Pat. No. 6,223,599 B1 asserted benefits of harmonic signals which would depend on short wavelength propagation.
Zhu and Steinberg discuss scattering issues (Zhu and Steinberg, xe2x80x9cModeling, measurement and correction of wavefront distortion produced by breast specimens, 1994 Ultrasonics Symposium, p1613, IEEE). Presumably this relates to diffraction processes, though their formulations (equations 1 and 2 of above reference) differ from the traditional approach to scattering. In any case, their small particle scattering model is insufficient to elucidate coherent aspects of scattering. Physiology shows that typical structure of breast tissue, as an example, includes lumps that are well beyond the size where the small particle model is valid. While Zhu and Steinberg consider such larger lumps (FIG. 5 of above reference), they consider these in terms of refraction alone.
Acoustic holography has been made to operate on the basis that when a general illuminating wave in a medium encounters irregularities in that medium, secondary waves are produced that can be individually determined and used to produce images.
The concept of an incident field and a scattered field is commonly used in physics and engineering to formulate problems where localized impedance and attenuation irregularities disturb waves. However, it appears that the general approach to correcting aberrations caused by wave speed variations is to find ways to reverse wave speed variations, as with eyeglasses. This approach seems to have dominated efforts by the academic and industrial ultrasonic research community to solve this problem for 15-20 years.
A general theory of aberrations in inhomogeneous media has been developed that formulates the effect of an aberration as a combination of an ideal wave and distortion waves. The aberrations are contained in a material that is otherwise homogeneous, to form the inhomogeneous media. This theory accommodates localized disturbances that include wave propagation speed variations as well as attenuation variations and impedance variations. An analysis method based on this formulation of the aberration problem has been used to understand aberration effects and to compute the characteristics of ideal waves and distortion waves as they undergo forward propagation processes. Apparatus was then envisioned that is structured to suppress the distortion waves. Rather than adjusting for wave speed variations as is typically done, the resulting devices seek to measure and eliminate distortion waves to leave ideal waves approximately intact. This is a unique type of spatial filter.
An example of such apparatus is a wave propagation device that includes an intended wave source of an initial spherical wave field, and a first receiving aperture. The initial spherical wave field is an ideal wave that propagates from source to the first receiving aperture, in a medium that contains irregularities that are distributed over a volume of the medium. The irregularities distort the ideal wave and each distortion results in a wave field that is equivalent to an additive combination of at least one distortion wave and a continuation of the ideal wave. Waves are sensed over the first receiving aperture surface to produce first received signals. Forward propagation is provided that allows any distortion wave to expand as a secondary spherical wave. This forward propagation takes place in a clear medium that allows any such distortion waves to expand without further distortion. It is then possible to measure the expanded distortion waves using a second receiving aperture device. Distortion wave measurements are then used to determine correction signals that are applied to the first received signals to eliminate effects of the distortion waves. Then a summation of corrected first received signals is carried out that focuses on the source of the initial spherical wave field. This summation now operates on signals that are caused only by the ideal wave. Reception of signals from the intended wave source is thus enhanced. The process works for many irregularities in the medium.
Importance of the invention increases significantly in the presence of a displaced, second source of a second spherical wave field that is also an ideal wave. It is critical in precise resolution devices to reject signal from this second source. This second ideal wave would be rejected by the focus operation using uncorrected signals, but a second set of distortion waves that it engenders would not. It is necessary to reject this second set of distortion waves so as to reject any signal that comes from the second source. Measurement of distortion waves is still done with forward propagation, but the presence of the second ideal wave complicates the process. An efficient way to control the second ideal wave, is to continue the forward propagation process where a focusing device is inserted to focus ideal waves to small spots of high intensity. The distortion waves would be flattened in the process. The flat distortion waves would be sensed by a flat transducer, which would be steered to locate sources of distortion waves, and thus locate corresponding signal points where corrections would be applied. The focused ideal waves would be subjected to a process, such as clipping in the electronic circuits, so that their intensity would be greatly reduced and their impact on measurement of distortion waves would be minimal.
Multiple undesired sources would be handled in this manner.
The flat distortion waves could alternatively be sensed using optical scanning, as done in acoustic holography. Such optical sensing offers a very efficient beamforming process. The continuation of the forward propagation process could alternatively be established by an additional receiver array and relay transmitter array. A subtraction technique could also be used to contend with the multiple sources, where signals from the first receiver would be beamformed to measure the set of unwanted sources to determine an approximate correction which would minimize affects of these unwanted sources.